High quality back-illuminated sign assemblies are widely used on theater marquees and entrances for first run motion pictures, and in other applications where a sign is required that is especially attractive in the evening. When used at first run motion picture theaters, such signs may be changed every few weeks. The sign maker may receive only short notice from the motion picture distributor that several hundred or several thousand signs of the rear illuminated type are required for advertising a new motion picture. If the sign that fits in a lightbox has to be completely discarded after each use, then the cost of the signs may be high and only a thin and "flimsy" sign may be provided. If thin material is used to minimize cost, then the outdoor signs may not withstand large gusts of wind. A sign and construction therefor method which enabled the rapid and low cost changing of the display of back illuminated signs, would be of considerable value.